In lasers such as semiconductor lasers used for optical communications and solid lasers used for laser processing etc., the laser oscillation is destabilized if light reflected by an optical surface or a work surface located outside a laser resonator returns to a laser element. The destabilized oscillation results in noise in a signal in the case of optical communications, or may result in destruction of the laser element in the case of a laser for processing. For this reason, an optical isolator is used for blocking such reflected optical feedback to prevent the reflected optical feedback from returning to the laser element.
In the meantime, fiber lasers have recently attracted attention as an alternative to the YAG laser (a laser for processing). Conventionally, a terbium gallium garnet crystal (hereinafter, referred to as a TGG) or a terbium aluminum garnet crystal (hereinafter, referred to as a TAG) has been used as a Faraday rotator used in an optical isolator in the fiber lasers.
However, the TGG and the TAG each have a small Faraday rotation coefficient per unit length. Hence, in order to obtain a polarization rotation angle of 45 degrees so as to function as an optical isolator, the TGG or the TAG needs to have a long optical path length, and hence a large crystal has been used. Moreover, in order to obtain a high optical isolation, it is necessary to apply a uniform and large magnetic field to the crystal. Hence, a strong and large magnet has been used. For this reason, the optical isolator is large in size. Moreover, because of the long optical path length, the shape of beams of the laser may be distorted in the crystal, which necessitates an optical system for correcting the distortion, in some cases. Moreover, since the TGG is expensive, there is a demand for a small and inexpensive Faraday rotator.
On the other hand, great reduction in size can be achieved by using, in an optical isolator of this type, a bismuth-substituted rare-earth iron garnet crystal film (hereinafter, referred to as an RIG), which is used exclusively in the field of optical communications. However, an RIG is known to deteriorate in performances when the wavelength of light used is shortened to around 1.1 μm, which is employed for a laser for processing, because a large amount of light is absorbed by iron ions, and this light absorption causes rise in temperature.
In this respect, methods have been proposed to solve the problem of rise in temperature in an RIG. For example, Patent Documents 1 and 2 describe a method in which a gadolinium gallium garnet substrate (hereinafter, referred to as a GGG substrate), which is an RIG growth substrate to be removed by grinding in ordinary cases, is left unremoved to facilitate dissipation of heat generated in an RIG. Moreover, a method (Patent Document 3) has been also proposed which uses a substrate, such as sapphire, having a high thermal conductivity and being conventionally used as a heat dissipation substrate.
However, any of these approaches is a mere technology for dissipation of heat generated in an RIG, and any of these approaches does not reduce the light absorption. Hence, there is a demand for a technology for reducing the amount of heat generated in an RIG by reducing the light absorbed by the RIG itself.
Here, it has been known that the above described iron ions contained in an RIG absorb light of about 1 μm, which is a wavelength of a laser for processing. However, iron is an important element which produces the Faraday effect in an RIG. Reduction in iron component results in increase in film thickness of the RIG necessary for achieving a Faraday rotation angle of 45°, which an optical isolator is required to have. Hence, in such a case, the reduction in amount of light absorbed in an RIG cannot be achieved, after all.
In this respect, as a technology for reducing the light absorption in an RIG at wavelengths around the 1 μm region, a method has been proposed in which the light absorbed by the iron ions is shifted toward shorter wavelengths by using, as an RIG growth substrate, a non-magnetic garnet substrate having a larger lattice constant instead of a (CaGd)3(ZrMgGa)5O12 substrate (hereinafter, referred to as an SGGG) having a lattice constant of 1.2497 nm and being widely used from the past in general. For example, Patent Document 4 describes an example in which an RIG was grown by using a Gd3(ScGa)5O12 substrate (hereinafter, referred to as a GSGG) having a lattice constant of 1.256 nm. Meanwhile, each of Patent Documents 5 and 6 describes an example in which an RIG was grown by using a Sm3(ScGa)5O12 substrate (hereinafter, referred to as an SSGG) or a La3(ScGa)5O12 substrate (hereinafter, referred to as an LSGG) having a lattice constant in the range from 1.264 to 1.279 nm.
In addition, any of these technologies is a method in which an RIG is grown by using a non-magnetic garnet substrate having a larger lattice constant than that of the conventionally used SGGG. By these methods, light absorbed by iron ions contained in an RIG is shifted toward shorter wavelengths, so that the amount of light absorbed is reduced.
However, when the thickness of the RIG of Patent Document 4 grown by employing a Gd3(ScGa)5O12 substrate (GSGG) having a lattice constant of 1.256 nm is adjusted to achieve a Faraday rotation angle of 45°, the absorption loss at a wavelength of 1.05 μm is about 1 dB, and the RIG does not have a sufficiently low loss. On the other hand, the RIGs of Patent Documents 5 and 6 grown by employing an SSGG or an LSGG having a lattice constant in the range from 1.264 to 1.279 nm do have absorption losses of 0.6 dB or less at a wavelength of 1.064 μm. However, since the SSGG and the LSGG are, in reality, difficult to obtain stably in the market, neither the SSGG nor the LSGG can be used as a substrate industrially.